1. Field
This disclosure is generally related to percutaneous or minimally invasive surgery, and more particularly to percutaneously deployed medical devices suitable for constricting tissue or a bodily orifice such as a mitral valve
2. Description of the Related Art
Cardiac surgery was initially undertaken only by performing a sternotomy, a type of incision in the center of the chest, which separates the sternum (chest bone) to allow access to the heart. In the previous several decades, more and more cardiac operations are performed using a percutaneous technique, which is a medical procedure where access to inner organs or other tissue is gained via a catheter.
Percutaneous surgeries benefit patients by reducing surgery risk, complications, and recovery time. However, the use of percutaneous technologies also raises some particular challenges. Medical devices used in percutaneous surgery need to be deployed via narrow tubes called catheter sheaths, which significantly increase the complexity of the device structure. As well, doctors do not have direct visual contact with the medical tools used once they are placed within the body, and positioning the tools correctly and operating the tools successfully can often be very challenging.
One example of where percutaneous medical techniques are starting to be used is in the treatment of a heart disorder called mitral regurgitation. Mitral regurgitation is a condition in which blood flows backward from the left ventricle into the left atrium. The mitral apparatus is made up of four major structural components and includes the annulus, the two leaflets, the chordae and the papillary muscles. Improper function of any one of these structures or in combination can lead to mitral regurgitation. Annular dilation is a major component in the pathology of mitral regurgitation regardless of causes and is manifested in mitral regurgitation related to dilated cardiomyopathy and chronic mitral regurgitation due to ischemia.
The mitral valve is intended to prevent the undesired flow of blood from the left ventricle into the left atrium when the left ventricle contracts. In a normal mitral valve, the geometry of the mitral valve ensures the cusps overlay each other to preclude the regurgitation of blood during left ventricular contraction and thereby prevent elevation of pulmonary vascular pressures and resultant symptoms of shortness of breath. Studies of the natural history of mitral regurgitation have found that totally asymptomatic patients with severe mitral insufficiency usually progress to severe disability within 5 years.
At present treatment consists of either mitral valve replacement or repair. Both methods require open heart surgery. Replacement can be performed with either mechanical or biological valves and is particularly suitable when one of the mitral cusps has been severely damaged or deformed. The mechanical valve carries the risk of thromboembolism and requires anticoagulation with all of its potential hazards, whereas the biological prosthesis suffers from limited durability. Another hazard with replacement is the risk of endocarditis. These risks and other valve related complications are greatly diminished with valve repair. Mitral valve repair is theoretically possible if the mitral valve leaflets are structurally normal but fail to appropriately coapt because of annular dilatation and/or papillary muscle dysfunction. Various surgical procedures have been developed to improve coaptation of the leaflet and to correct the deformation of the mitral valve annulus and retain the intact natural heart valve function. These procedures generally involve reducing the circumference of the posterior mitral leaflet annulus (lateral annulus) where most of the dilatation occurs. The annulus of the anterior leaflet (septal annulus) does not generally dilate because it is anchored to the fibrous skeleton at the base of the heart. Such techniques, known as mitral annuloplasty, typically suture a prosthesis around the base of the valve leaflets shortening the lateral annulus to reshape the mitral valve annulus and minimize further dilation. Different types of mitral annuloplasty prostheses have been developed for use in such surgery. In general, such prostheses are annular or partially annular shaped and may be formed from rigid or flexible material.
Mitral valve surgery requires an extremely invasive approach that includes a chest wall incision, cardiopulmonary bypass, cardiac and pulmonary arrest, and an incision on the heart itself to gain access to the mitral valve. Such a procedure is expensive, requires considerable time, and is associated with high morbidity and mortality. Due to the risks associated with this procedure, many of the sickest patients are denied the potential benefits of surgical correction of mitral regurgitation. In addition, patients with moderate, symptomatic mitral regurgitation are denied early intervention and undergo surgical correction only after the development of cardiac dysfunction. Furthermore, the effectiveness of such procedures is difficult to assess during the procedure and may not be known until a much later time. Hence, the ability to make adjustments to or changes in the prosthesis function to obtain optimum effectiveness is extremely limited. Correction at a later date would require another open heart procedure.
In an attempt to treat mitral regurgitation without the need for cardiopulmonary bypass and without opening the chest, percutaneous approaches have been devised to repair the valve or place a correcting apparatus for correcting the annulus relaxation. Such approaches make use of devices which can be generally grouped into two types:                devices deforming (mainly shortening) the coronary sinus        devices pulling together two anchor points in order to affect the mitral valve, one of the anchor points can be the coronary sinus (typically using a wire that is pulled and secured).        
Neither approach emulates the current “gold standard” in mitral valve repair annuloplasty using a ring the completely encircles the mitral valve. Both approaches suffer from several problems as a result of attempting to reshape the mitral annulus using an alternative method. Devices that deform the coronary sinus, while suitable for percutaneous procedures, are not effective in controlling the leakage of the mitral valve as the forces are not applied from the correct opposite sides of the valve, which are the lateral annulus and the septal annulus. The prior art devices of the second type are not easily adapted to a percutaneous procedure. In order to achieve shortening in the direction connecting the lateral annulus to the septal annulus the anchor points have to be located along this line, so pulling them together will affect the desired direction of shortening. Pulling applied along a different direction will distort the mitral valve but will not achieve the optimal approximation of the two leaflets.
Thus, there is a need for methods and apparatus that enable the ability to create a full ring mitral annuloplasty via a percutaneous or intravascular procedure.